Presentation on theme: "Introduction to Long-Throated Flumes and Broad-Crested Weirs"— Presentation transcript:
1 Introduction to Long-Throated Flumes and Broad-Crested Weirs The term long-throated flume describes a broad class of critical-flow flumes and broad-crested weir devices used to measure flow in open channels.
2 Critical-Flow Measurement Devices Flumes, sharp-crested weirs, broad-crested weirsProduce critical-depth flow in a control sectionFroude number = V/(gD)1/2=1Critical depth occurs at locations where the downstream depth does not “hold the flow back”Minimum specific energy for a given flowMaximum flow for a given specific energyShallow-water waves cannot travel upstreamTailwater does not affect headwater elevationFlow rate through the critical section is a function of the upstream head, acceleration of gravity, and the control section size
3 Traditional Critical-Flow Devices Most critical-flow devices have curvilinear, three-dimensional flow fields in the control sectionAll such devices require laboratory calibrationFlumesParshall flumes, cutthroat flumes, H-flumes, etc.WeirsV-notch weirs, Cipoletti weirs, contracted and suppressed rectangular weirs, etc.
4 Long-Throated Flumes and Broad-Crested Weirs Long-throated flumes have one-dimensional flow in the control section -- Long-throated means long enough to eliminate lateral and vertical contraction of the flow at the control section…streamlines are essentially parallelCan be calibrated using well-established hydraulic theoryNo laboratory testing neededCalculations are iterative, so computer models that do the calculations have made long-throated flumes reasonable to implement in recent years
5 Submergence of Flumes and Weirs h2/h1 or hb/ha is the submergence ratioSharp-crested weirsNO SUBMERGENCE ALLOWEDParshall flumeSome submergence allowedLong-throated flumeMost submergence allowed
6 Submergence of Parshall and Other Short-Throated Flumes Some submergence allowedModular limit varies with flume size (50-80 percent)Submergence correction needed when above modular limit (no longer a true critical-flow device)…even with correction, accuracy suffers, especially above 90 percent submergence
7 Submergence of Long-Throated Flumes and Broad-Crested Weirs Highest modular limit (lowest head loss) of any critical-flow deviceModular limit varies, but can be predicted using WinFlumeTypically percentNo submergence correction needed as long as submergence is less than modular limitFlow rate is always a function of ONLY the upstream water levelCannot correct for submergence if modular limit is exceeded
8 Long-throated flumes are the measurement device of choice for many applications. Advantages include: Rating tables with error of less than 2% in the computed discharge can be computed for any combination of prismatic control section and an arbitrarily shaped approach channelIf the throat is horizontal in the direction parallel to the flow, accurate rating tables can be computed using as-built dimensionsThroat can be any shape in the direction perpendicular to the flow, allowing the complete range of discharges to be measured with good precisionRequired head loss across the flumes is minimal (modular limit is high)Long-throated flumes can be operated in free-flow with greater submergence than all other critical-flow devices, and the submergence limit (modular limit) and the associated head loss requirement can be determined using the WinFlume programWith properly constructed gradual converging transition, there is virtually no problem passing floating debrisCan be designed to pass sediment transported by channels having subcritical flowEconomical to constructVery adaptable to installation in existing canals
9 Principal Design Issues Upstream channel - 3 issuesUniform, fully-developed flow conditions approaching structure so that hydraulic theory is applicableStable water surface to allow accurate measurement of sill-referenced headEffect that the flume may have on upstream structures and freeboard in upstream channelFlume contraction - 2 issuesEnsure that flume is not submerged by tailwater (contraction must be enough to force critical depth)Ensure that contraction produces enough head to make an accurate flow measurementComponent lengths - must meet “long-throated” criteria
10 Lengths of Flume Components Throat section length, L0.07<H1/L<0.7 for ±2% uncertainty0.05<H1/L<1.0 for ±4% uncertaintyFloor and sidewalls of converging transition2.5 to 4.5:1 transition slopeDiverging transitionNo flatter than 10:1Gaging station location (approach channel length)> H1 upstream of start of converging transition(2 to 3) * H1max from start of throat
11 Throat Section Shape Selection ConstructabilityRange of Flows to be Measured
12 Typical Flume/Weir Configurations Sill in a concrete-lined canalRectangular-throated flumes for earthen canalsTriangular-throated flumes for natural channelsFlumes in circular pipesPortable and temporary flumes
25 Flume Design & Selection Pre-computed flume designs can be chosen using tables in the Water Measurement ManualDesigns can be developed using the WinFlume computer programAllows for customizationProvides best rating table accuracySimplifies checking of design
33 WinFlume SOFTWARE FOR THE DESIGN AND CALIBRATION OF LONG-THROATED FLUMES AND BROAD-CRESTED WEIRS Water Resources Research LaboratoryDenver, ColoradoInternational Institute for Land Reclamation & ImprovementWageningen, The NetherlandsU.S. Water Conservation LaboratoryPhoenix, Arizona
34 Evolution of Long-Throated Flume Design Software Batch oriented FORTRAN codes developed by ARS at U.S. Water Conservation Laboratory, Phoenix.Interactive program written in Clipper for DOS-based computers, available since early 1990’s (FLUME 3.0). Joint effort of ARS and ILRI.WinFlume program written in Visual Basic for Windows 95/NT environment. This is an upgrade of FLUME 3.0, and is a joint effort of Reclamation and ARS, with programming work being performed by Reclamation.
35 WinFlume Usability Improvements Designed for Windows 95 / Windows NTGraphical user interfaceFreedom from DOS memory limitations of FLUME 3.0Supports all Windows-compatible printers and plottersNew *.FLM file format simplifies transfer of flume designs among usersBackward compatible (loads FLUME 3.0 designs)Improved plotting of discharge curvesOutput to printer, file-on-disk, or system clipboardOnline help system
36 New Hydraulic Analysis & Design Features New design module simplifies algorithms used to find an acceptable flume design…solves problem of the software not finding acceptable design in some complex casesAdditional parameters in rating tables(e.g., actual tailwater and submergence ratio at all points in rating table)Direct interrogation of rating tables to determine applicable range of hydraulic errors or warnings
37 New Output Features Full-scale plotted wall gauges Can be provided to fabricator for construction of durable wall gauges graduated in head or discharge units, adjusted for installation on sloped canal bankOn-screen preview of finished wall gaugePlotted continuously on roll-feed plotters, or in sections with match marks on page-type printers (e.g. laser printers)Ditchrider’s rating tableAlternative equation forms in curve-fitting module:
38 Calibrating an Existing Structure Define geometry of canal and flumeProvide hydraulic data and other propertiesConstruction materialTailwater conditionsGenerate outputRating tables and curvesCurve-fit equation for data loggerWall gage data and/or plot
39 Designing a New Structure Define canal geometry and initial flume control section shapeProvide hydraulic data, canal/flume properties, design requirementsConstruction materialTailwater conditionsWater level measurement method and required flow measurement accuracyRequired freeboard in upstream channelEvaluate designs and choose final designGenerate output
40 Design Criteria, Methods, Objectives Primary Design Criteria (4)Maintain necessary freeboard at maximum flowMaintain modular (critical) flow at minimum and maximum flow (ensure free flow)Froude number at maximum flow must be less than 0.5Secondary Design Criteria (2)Control section must produce sufficient head to provide a user-specified level of discharge-measurement precision at both minimum and maximum flow, based on the precision of the device used to measure upstream water level.Control Section Adjustment MethodsRaise sill heightRaise entire throat sectionRaise inner section of throat (used with complex cross-sections)Adjust lateral contraction by changing bottom width of throat section or changing diameter or focal distance of circular or parabolic sections (only option for movable-crest flumes)Head Loss ObjectivesMinimum head lossMaximum head lossIntermediate head lossMatch head loss to drop in canal invert at site
42 WinFlume’s Design Procedure User chooses one of the four methods of contraction change, and an increment at which to evaluate designs (e.g. evaluate designs at sill height increments of 0.1 ft).WinFlume brackets the range of possible designs by evaluating flume performance at the maximum design flow:The maximum possible throat-section contraction is that needed to produce a maximum upstream water level equal to channel depth.The minimum contraction is that which produces an upstream Froude number of 0.5 or less at maximum discharge, and an upstream water level that is at least as high as the downstream tailwater at maximum discharge.WinFlume builds and evaluates designs of “virtual” flumes between the lower and upper contraction limits at the interval specified by the user.
43 Design Procedure (continued) Results are presented to the user, who may choose to accept one of the designs or discard the results of the analysis.Only designs meeting the four primary design criteria (freeboard, Froude number, no submergence at minimum and maximum flow) are presented to the user, unless there are no acceptable designs.Designs that meet the four primary criteria, but do not meet measurement precision requirements may be improved by the user by specifying a better water level measurement method.Acceptable designs that have minimum head loss, maximum head loss, intermediate head loss, or head loss matching the bed drop at the site are listed in the output, along with any other acceptable designs. The user can evaluate the different head loss characteristics of the possible designs and then choose the design that best meets their needs.
44 If An Acceptable Design Is Not Found On The First Trial If contraction increment is too large, or if design criteria are too limiting, no acceptable design may be found.WinFlume searches for two adjacent designs for which the unsatisfied criteria in each design are satisfied in the adjacent design.An acceptable design may exist between those two designsAnalysis is repeated using a smaller increment of contraction change within that range.If no region of acceptable designs is found, then the results of the analysis are presented to the user, with suggestions for how to relax the design criteria or change the initial design so that an acceptable design can be found.
45 HOW TO OBTAIN WINFLUME http://www.usbr.gov/wrrl/winflume The WinFlume program is available on the World Wide Web at:There are 16-bit and 32-bit versions available that are appropriate for Windows 3.1x, Windows 95, Windows 98, and Windows NT systems.This work has been funded by the U.S. Bureau of Reclamation’s Water Conservation Field Services Program.
46 Off-season drainageFor most irrigation-system flumes, flow through a 2-inch or smaller pipe is negligible compared to flow over structureLarger drain pipes can be plugged with rags or closed with valves on the downstream side while operating